Method and system for the treatment of water and fluids with chlorine dioxide

ABSTRACT

Embodiments of the invention relate generally to methods and systems for treating aqueous systems associated with industrial wastewater applications and gas and crude oil drilling, pumping and production to reduce or eliminate contamination. In one embodiment, a method includes: an aqueous volume having an initial oxidant demand, an oxygen-containing oxidant and at least one chlorine oxide at substoichiometric amounts in an amount sufficient to eliminate the oxidant demand. The system includes: an aqueous treatment system comprising a containment system; at least one apparatus for introducing an oxygen-containing oxidant; and at least one apparatus for introducing at least one chlorine oxide into said containment system at controlled, substoichiometric quantities.

FIELD OF THE INVENTION

The recited claims relate generally to methods and systems for treatingaqueous systems associated with industrial wastewater applications andgas and crude oil drilling, pumping and production, including but notlimited to hydraulic fracturing. More particularly, the recited claimsrelate to an improved method and system of treating contaminatedfracturing fluids, produced water, flowback water, source water or otherindustrial aqueous fluids to reduce or eliminate contamination.

INTRODUCTION

Wastewater associated with hydraulic fracturing and the production ofcrude oil, i.e. oilfield water, generally consists of two primarysources: flow-back water and produced water. The reuse of these watersis typically difficult due to high contaminant and bacterial loading.More specifically, oilfield water and fracturing fluids (or frac water)can be contaminated with, for example, bacteria, naturally-occurringorganics in the formation, organic treatment chemicals (such asviscosifiers, emulsion stabilizers, etc), and production chemicals (suchas scale reducers, friction reducers, anti-corrosive chemicals, pHmodifiers, etc.), and/or other contaminants that result in a highpercentage of TDS. The presence of these contaminants can interfere withlater re-use of the water, storage and/or disposal (e.g. injection intodisposal wells or sent to municipal treatment facilities).

For example, municipal treatment facilities are facing increasingregulatory requirements for wastewater associated with hydraulicfracturing and satisfying these requirements is costly. Similarly, tothe extent contaminated frac and oilfield waters are stored in oilfieldpits, open pools, or lagoons, high residual polymer levels and solidsloading within the pits can contribute to high hydrogen sulfideproduction, causing safety and environmental concerns.

More recently, producers are shifting to closed loop systems as thepreferred method of handling flowback and produced waters (i.e. reusingthese waters in subsequent operations). As such, the water used forhydraulic fracturing operations is often a combination of producedand/or flowback water, surface water and/or municipal water (also knownas “commingled water”). Successful pre-treatment of the contaminatedwater in the storage pits and tanks allows commingled water to be madeup of a larger percentage of flowback water and other frac fluids thanit would otherwise, and in turn, provide for reduced disposal costs,fresh water costs, and lower water use concerns. Thus, the methods andsystem disclosed herein will help to reduce and/or effectively eliminatebacteria-contaminated and/or organic chemicals in order to ultimatelyreduce the water footprint associated with hydraulic fracturing andcrude oil production, as well as other industrial wastewaterapplications.

Various methods and systems for the treatment of oilfield and fracwastewater and source water have been explored and are known in the art.One of the methods includes the use of chlorine dioxide for thetreatment of pits, lagoons and tanks storing oilfield waters andfracturing fluids. Chlorine dioxide's unique chemical and physicalproperties make it ideal for use in treatment of fracturing fluids. Asan oxidant, it is able to penetrate hydrocarbons and break emulsionsallowing for the separation and recovery of hydrocarbons, as well as thereduction and/or elimination of biological contamination. Because of itsspecificity, its oxidation power can be directed at contaminates such assulfides and residual polymers without creation of undesirableby-products and, unlike bleach or chlorine, chlorine dioxide does notform chlorination by-products that can cause operational orenvironmental concerns.

Chlorine dioxide (and/or chlorite), however, is heavily regulated andcaution is necessary in its generation, handling and storage.Furthermore, it can be very costly depending on the chlorine dioxidedemand of the wastewater and/or source water. The various embodiments ofthe invention use the oxidative power of chlorine dioxide together withoxygen (or air) to achieve an unexpected increase in efficiency andcapacity for treatment for these waters, thereby substantially loweringthe amount of chlorine dioxide required to substoichiometric amounts. Inaddition, and according to some of the various embodiments of theinvention, a combination of chlorine dioxide disinfection andoxygenation is used to provide a faster-acting treatment for wastewater.Such methods and systems result in increased chlorine dioxide capacityand increased efficiency in relation to volumes of water treated, whichprovides for reduced chemical usage, reduced energy, and reducedeffluent, which in turn results in a reduced burden on the environmentand reduced cost.

In accordance with one or more of these embodiments, the use of chlorinedioxide to treat pits, lagoons and tanks storing fracturing fluids andoilfield wastewater has the potential to provide reduced treatmentcosts, fluid disposal costs and make-up water purchases (by allowing forgreater reuse of the oilfield wastewater), and reduced environmental andsafety concerns. Chlorine dioxide can also be used for the pretreatmentand disinfection of fracturing fluids prior to their use in crude oilproduction and/or hydraulic fracturing operations, including but notlimited to surface water, produced water, municipal water, flowbackwater, or any combination thereof.

Accordingly, it is desirable to provide methods and systems for thetreatment of wastewater associated with gas and crude oil drilling,pumping and production, including but not limited to hydraulicfracturing, as well as other industrial applications, that alleviateseveral of the problems associated with existing treatments. It is alsodesirable to provide methods and systems for improved treatment offracturing fluids.

SUMMARY

In one aspect, the invention relates to a method for treating an aqueoussystem, comprising providing an aqueous volume having an initial oxidantdemand; introducing an oxidant, wherein said oxidant comprises oxygen,air, ozone, or a combination of the same; combining the aqueous volumeand oxidant and allowing the oxidant to lower the initial oxidant demandto a reduced oxidant demand; providing at least one chlorine oxide; andcombining the aqueous volume and a substoichiometric quantity of atleast one chlorine oxide in an amount sufficient to eliminate thereduced oxidant demand, wherein said at least one chlorine oxidecomprises chlorine dioxide, chlorite, or a combination of the same andsaid substoichiometric quantity is less than the reduced oxidant demandin said aqueous volume.

In another aspect, the invention also relates to an aqueous treatmentsystem comprising a containment system comprising an aqueous volume; atleast one apparatus for introducing an oxidant into said containmentsystem in controlled quantities and at a controlled flow rate, whereinsaid oxidant comprises oxygen, air, ozone or combinations thereof and atleast one apparatus for introducing at least one chlorine oxide intosaid containment system at controlled, substoichiometric quantities toachieve a chlorine dioxide residual of at least 0.1 mg/l, wherein saidchlorine oxide comprises chlorine dioxide, chlorite, or combinationsthereof.

In yet another aspect, the invention also relates to a method forreducing, inactivating, destroying, or eliminating oxidant demand,sulfur compounds, bacteria or a combination thereof from an aqueousfluid volume or stream comprising the steps of introducing a chlorineoxide and an oxidant, wherein the chlorine oxide is added atsubstoichiometric amounts as compared to a predetermined demand for saidchlorine oxide and wherein said oxidant is selected from the groupconsisting of oxygen, air, oxygen-enriched air, ozone, and combinationsthereof and said chlorine oxide is selected from the group consisting ofchlorine dioxide, chlorite, and combinations thereof.

The invention further relates to a method for treating an aqueoussystem, comprising introducing an oxidant into an aqueous volume at aflow rate that avoids off-gassing of volatile reductants from theaqueous volume prior to introducing chlorine dioxide into the volume,wherein said oxidant is selected from the group consisting of oxygen,air, oxygen-enriched air, ozone, and combinations thereof, and whereinthe oxidant provides synergistic oxidation activity in the presence ofthe chlorine dioxide such that the chlorine dioxide is introduced atsubstoichiometric amounts as compared to a predetermined chlorinedioxide demand.

GLOSSARY

The following terms as used herein have the following meanings:

Demand—The amount of chlorine dioxide (or other oxidant) consumed bybackground, reactive impurities (both inorganic and organic materials)in a given sample of wastewater (i.e. oilfield water), fracturing fluid,treatment or other target fluids. Chlorine dioxide demand is determinedby subtracting the amount of chlorine dioxide remaining after aspecified time from the amount of chlorine dioxide initially added to asystem.

Free Residual or Residual—The amount of chlorine dioxide (or otheroxidant) present at a given time to react with biological species afterbackground contaminants (or “demand”) have been converted. In otherwords, the amount of chlorine dioxide (or other oxidant) available forbacterial control.

Biocide—chemical agent capable of killing living microorganisms, oftenin a selective way (also referred to as bactericides or antimicrobials).

Biological Contamination—any living microorganism or by-product of aliving microorganism found in wastewater (i.e. oilfield water),fracturing fluids, treatment fluids, source water or other targetfluids.

Biocidally-Effective Amount—An amount that will control, kill orotherwise reduce the bacterial or microbial content of the wastewater(i.e. oilfield water), fracturing fluids, treatment fluids, source wateror other target fluids at issue.

Well Fluid, Fracturing Fluid or Frac Fluid—Any fluid used in any of thedrilling, completion, work over and production of subterranean oil andgas wells. It generally includes a source (or raw, or base) water feed(e.g. Frac Water) plus any additives.

Frac Water—Raw water feed used in hydraulic fracturing process from anysource, including but not limited to surface water, municipal water ortreated flowback or produced water.

Produced Water—Water that is naturally occurring within a subterraneanformation that is produced to the surface either as part of a hydraulicfracturing or crude oil operation

Flowback Water—Recovered fracturing fluids that flow back to the surfaceafter being pumped down into a subterranean formation as part of ahydraulic fracturing or crude oil operation.

Oilfield Water—As used herein, includes production water, flowback waterand other fluids that are the by-products of crude oil production,hydraulic fracturing, or other petroleum production processes.

Furthermore, as used herein, the words “comprise”, “has,” and “include”and all grammatical variations thereof are each intended to have anopen, non-limiting meaning that does not exclude additional elements orparts of an assembly or structural element.

The features of the present invention will be readily apparent to thoseskilled in the art upon a reading of the description of the embodimentsthat follows.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, and any alterations and furthermodifications in the illustrated embodiments, and any furtherapplications of the principles of the invention as illustrated thereinas would normally occur to one skilled in the art to which the inventionrelates are contemplated an protected.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include” (and any formof include, such as “includes” and “including”), and “contain” (and anyform contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises”, “has”,“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises”, “has”, “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

Hydraulic fracturing and other oil field drilling and productionprocesses require large quantities of water and, in turn, produce largequantities of wastewater. Additionally, many other types of industrialor commercial operations rely on large quantities of water and producelarge quantities of wastewater, all of which needs to be treated. Theseindustries include, but are not limited to, agriculture, chemical,pharmaceutical, mining, metal plating, textile, brewing, food andbeverage processing, and semiconductor industries. The presence ofbiological contamination and other organic contaminants results indecreased efficiency and can cause damage (i.e. corrosion, blockages,growth of harmful bacteria). Similarly, waters that have high residualorganic or biological contamination are unsuitable for use in oilfieldoperations and need to be treated prior to being injected undergroundand introduced into a subterranean formation.

In accordance with the embodiments of the invention, chlorine dioxidecan be used to treat oilfield water (including production water,flow-back water and surface water) in order to reduce both thebiological load and to aid in the breakdown of residual organiccontamination in the water. For example, although not limiting, one ormore embodiments of the present invention may be used for the treatmentof produced or flowback water prior to disposal or reuse. Both producedand flowback water tend to have substantial biological contamination, aswell as a high load of organic contaminants (such petroleumhydrocarbons, oil and grease, diesel-related organics, BTEX), polymers(such as polyacrilamides), iron (Fe), transition metals, suspendedsolids, and other contaminants.

In one or more embodiments of the invention, and by way of example only,the methods disclosed herein can be used to treat produced or flow-backwater: 1) before the water is released back into the environment; 2)before the water is used for use in a subsequent hydraulic fracturingoperation; 3) as a pretreatment for frac water, including but notlimited to a pretreatment “on the fly”; 4) before the water is depositedin storage pits/tanks/lagoons; or 5) as part of a closed-loop oilfieldproduction system.

For example, one embodiment is a process for the disinfection andoxidation of wastewater and contaminated fluids, that provides for asubstantial reduction in the amount of chlorine dioxide required toremove oxidant demand and/or eliminate biological contamination, andeventually achieve a final residual of chlorine dioxide in the range ofabout 0.1 mg/l and 50 mg/l, thus making the wastewater suitable forre-use. In one embodiment, a combination of a chlorine oxide 30 and anoxidant 40 is introduced into a fluid stream 100. Fluid stream 100comprises wastewater 15, for example, a wastewater fluid stream from ahydraulic fracturing site, or any target fluid. Chlorine oxide 30comprises chlorine dioxide (30a), chlorite (30b) or a combinationthereof, which are introduced into stream 100. For example, chlorinedioxide can be introduced via education using a venturi 20, whereinventuri 20 is part of fluid stream 100 being treated, or other meanswell known in the art. As used herein, oxidant 40 can be air, oxygen,oxygen-enriched air, ozone, any chemical oxygen source or combinationthat is stable with chlorine dioxide (30a) and/or chlorite (30b), orsome combination of the same. In one or more embodiments of theinvention, oxidant 40 is introduced via direct injection into thewastewater in fine bubbles (i.e. air sparging,), a pressurized source80, an aerator, mechanical agitation, a diffuser, spraying, or educationvia venturi 20.

For example, a closed loop treatment system 150 is shown. In thisembodiment, closed loop treatment system 150 comprises a venturi 20, afluid stream 100 (e.g. wastewater 15) to be treated, storage vessel 50,chlorine oxide 30 and oxidant 40. In accordance with one embodiment ofthe invention as applied to the oilfield and fraccing industry, storage50 contains wastewater 15, which is supplied by a source 10. Source 10comprises a source of produced water, flowback water, surface water,municipal water, frac water, wastewater, or any combination thereof. Oneof ordinary skill in the art, however, will recognize that wastewater 15can be any water or target aqueous fluid that is contaminated (forexample, with organics and/or microorganisms) and is being recycled ortreated for reuse, storage and/or discharge back into the environment,regardless of industry. In one or more embodiments, the oxidant demandof the contaminants in wastewater 15, prior to treatment, is from about30 mg/l to about 5000 mg/l, preferably from about 50 mg/l to about 500mg/l. The oxidant demand comprises reducing agents including, but notlimited to, reduced sulfur compounds, biomass and other biologicalby-products, and reduced metals including but not limited to iron (Fe)II.

In one embodiment, venturi 20 is used to both generate and introducechlorine oxide 30 (i.e. chlorine dioxide (30a), and/or a combination ofchlorine dioxide (30a) and chlorite (30b)) into fluid stream 100 and,additionally, to then introduce oxidant 40. In a preferred embodiment, adrive fluid 33 for venturi 20 comes directly from storage vessel 50.Vessel 50 contains wastewater 15, i.e. the wastewater to be treated, ora combination of treated wastewater (or other target fluid) and thewastewater to be treated. One of ordinary skill in the art willrecognize, however, that drive fluid 33 can come from any availablewater source placed in line with system 150. In accordance with theinvention, storage vessel 50 is a tank, pit or pond, or any otherstorage means (e.g. reservoir, container, or lagoon) that stores, holds,transports or contains wastewater 15 from source 10.

In embodiments of the invention, chlorine oxide 30 and oxidant 40 areapplied at such a rate that the removal of volatile reductants (i.e.hydrogen sulfide) is via oxidation, rather than physical purging orstripping. By selecting an air flow rate that prevents or avoidsoff-gassing of the hydrogen sulfide (or other volatile reductants)present in wastewater 15, the reductants are oxidized in situ ratherthan purged. The goal is to add oxidant 40 to the fluid at a flow ratethat brings it into contact with the sulfides to allow oxidation tooccur. Thus, a flow rate that results in the addition of air beingviolent, and thus stripping the sulfides before they can oxidize, shouldbe avoided. The volume of vessel 50 (or fluid to be treated) willdirectly affect the range of flow rates that can be used to avoidoff-gassing/purging and, thus, the appropriate range is widespread. Forexample, a small tank would require a much lower air flow rate than adeep pond. However, one of ordinary skill in the art will be able todetermine the appropriate flow rate to avoid purging, or stripping, ofthe volatiles, depending on the volume, depth and/or size of vessel 50(or fluid to be treated), the treatment system and demand.

In embodiments of the invention, a diffuser 70 is used to introduceoxidant 40. In one or embodiments, oxidant 40 is added directly towastewater 15 near the bottom of vessel 50 and the resultant mechanicalaction is thereby used to enhance mixing of wastewater 15 within vessel50. If a single point introduction method is used, it is preferred thatthe injector be movable throughout the horizontal plane of vessel 50(not shown).

In one or more of the embodiments disclosed herein, chlorine oxide 30and oxidant 40 are introduced into wastewater 15 as follows. Initially,chlorine oxide 30 is introduced for a sufficient amount of time and at asufficient dosage to reduce the chlorine dioxide demand of thewastewater 15 by about 10 percent to about 20 percent. The amount oftime and dosage required will depend on the characteristics ofwastewater 15 (e.g. chlorine dioxide demand), the treatment system, andthe intended use or application. In this step, chlorine oxide 30 may bechlorine dioxide (30a), chlorite (30b) or a combination thereof. In oneor more embodiments, during this initial (or first) stage of treatment,chlorine oxide 30 comprises chlorite (30b) only. In embodiments wherechlorine oxide 30 comprises chlorite (30b) only, the step of introducingoxidant 40 (see below) may be (and, in many instances, is preferred tobe) performed simultaneously. On the other hand, because chlorinedioxide reacts as a free radical and, therefore, reacts almostinstantaneously, chlorine dioxide cannot be added at high rates orconcentrations at the same time as when a large volume of oxidant 40 isbeing added. Therefore, if chlorine oxide 30 comprises chlorine dioxide(30a) during this initial step (or a combination of chlorite (30b) andchlorine dioxide (30a)), oxidant 40 cannot be added at the same timeuntil all of the chlorine dioxide (30a) has converted to chlorite (30b)or, if performed simultaneously, oxidant 40 must be added at a rate lowenough to make sure any chlorine dioxide is not stripped, or purged,from wastewater 15 before it disperses through the fluid body.

Furthermore, in certain embodiments, caustic can be added either priorto treatment with chlorine oxide 30, or concurrently therewith, to raisethe pH of wastewater 15 to about 7-10. By introducing a higher pH forwastewater 15, contaminant metals (for example, iron (Fe)) will drop outof solution and the formation of certain metal complexes that tend toform in low pH will be avoided. In still other embodiments, the firststep of adding chlorine oxide 30 can be skipped, depending on thechlorine dioxide demand and the application/system at hand.

Next, oxidant 40 is introduced into wastewater 15. Oxidant 40 is addedat an appropriate dosage and period of time to achieve an overall dosageranging from about 20 mg/kg to about 2000 mg/kg of oxidant 40 to thetotal volume of wastewater 15 to be treated, with a more preferreddosage of about 20 mg/kg to about 1000 mg/kg of oxidant 40 to the totalvolume of fluid to be treated. Again, the amount of time and dosagerequired will depend on the characteristics of wastewater 15 (e.g.chlorine dioxide demand), the treatment system, and the intended use orapplication, together with other mechanical considerations known tothose of ordinary skill in the art. In one or more embodiments, theapplication of oxidant 40 consumes, in total, from about 10 percent toabout 90 percent of the total chlorine dioxide demand, preferably fromabout 60 percent to about 90 percent of the chlorine dioxide demand. Asdiscussed above, the step of introducing oxidant 40 can be performedsimultaneously with the first step of adding chlorine oxide 30, inparticular when chlorine oxide 30 is chlorite (30b) only during thefirst treatment step.

In the next step, chlorine oxide 30 is introduced at substoichiometricamounts until the target chlorine dioxide residual is reached. In thisstep, chlorine oxide 30 comprises one or more of chlorine dioxide (30a),chlorite (30b) or a combination thereof, and it does not have to be thesame as what was used for chlorine oxide 30 in the initial step. Forexample, in one preferred embodiment, chlorite (chlorine oxide 30b),oxygen (oxidant 40), and caustic (optional) are introducedsimultaneously at the beginning of the treatment process for a period oftime (i.e. about 15 to about 60 minutes), followed by substoichiometricquantities of chlorine dioxide (chlorine oxide 30a) until the targetresidual of chlorine dioxide is reached. The target residualconcentration of chlorine dioxide in the treated fluid or wastewaterdepends on the intended storage period prior to use. For example, forimmediate use as frac water in a hydraulic fracturing system, thedesired chlorine dioxide residual of fluid 200 is between about 0.1 mg/land about 20 mg/l, preferably between about 0.5 mg/l and about 5 mg/l.By way of further example, if the treated fluid is to be stored invessel 50 for several days or more, the target residual concentration ofchlorine dioxide should be between about 5 mg/l and about 50 mg/l,preferably between about 20 mg/l and about 50 mg/l. In one or moreembodiments, chlorine oxide 30 comprises chlorine dioxide (30a) onlyduring the last stage of the treatment process. During this treatmentstep, chlorine oxide 30 (in the form of chlorine dioxide (30a)) andoxidant 40 cannot be added to stream 100 at the same time.

In one or more embodiments of the invention, the total treatment timerequired for wastewater 15 to achieve oxidation and/or disinfection isless than 24 hours, preferably less than 8 hours, if storage (or vessel)50 is a tank, pit, pond, or lagoon. In still other embodiments, thetotal treatment time required for wastewater 15 to achieve oxidationand/or disinfection is less than about 60 minutes, and preferably lessthan about 15 minutes, if vessel 50 is a pipeline, or a combinationpipeline and a tank, such as would be used for “on the fly” operationsout in the field, when there is a limited residency time and treatedfluid 200 is to be used immediately.

In one embodiment, air injection is used to introduce oxidant 40 intovessel 50 via diffuser 70 and a pressurized source 71. In yet anotherembodiment, a chemical tank 80 is used to introduce oxidant 40. Inanother embodiment, diffuser 70 and pressurized source 71, placedin-line, are used to introduce oxidant 40 into stream 100. In stillother embodiments, treatment system 250 is, for example, afrac-on-the-fly treatment system or any other industrial water treatmentsystem that is placed in-line for immediate use. In one embodiment,oxidant 40is introduced in-line via venturi 20 from chemical source 72.In another embodiment, a chemical tank 80 is used in-line to introduceoxidant 40 into stream 100. In still another embodiment, air injectionis used to introduce oxidant 40 into stream 100 via diffuser 70 and apressurized source 71.

Any appropriate method of producing chlorine dioxide known in the artmay be used to generate chlorine dioxide suitable for use in the presentinvention. In general, chlorine dioxide solutions can be produced bytreatment of chlorite salt solutions (e.g. NaClO₂) with an acid solutionto produce acidic solutions that contain ClO₂, which can be then beflushed as a gas into water to produce aqueous ClO₂. Other precursorssuch as sodium chlorate can also be used.

Several chemical means of generating chlorine dioxide and theircorresponding chlorine dioxide precursor chemicals are known in the art,and the choice of suitable means and chemicals is within the abilitiesof those skilled in the art. Exemplary chemical means of generatingchlorine dioxide are disclosed in U.S. Pat. No. 4,689,169 (Mason etal.), U.S. Pat. No. 5,204,081 (Mason et al.), U.S. Pat. No. 5,227,306(Eltomi et al.), U.S. Pat. No. 5,258,171 (Eltomi et al.), U.S. Pat. No.5,965,004 (Cowley et al.), and U.S. Pat. No. 6,645,457 (Mason et al.)the disclosures of which are incorporated herein by reference.

In preferred embodiments, the chlorine dioxide should be of the highestpossible purity. More specifically, chlorine gas should be present inthe introduced chlorine dioxide gas at a level less than about 5%,preferably less than about 0.5%. For example, in a preferred embodiment,the present invention provides a process that comprises producingchlorine dioxide by using an apparatus such as a chlorine dioxidegenerator, e.g. as disclosed and claimed in U.S. Pat. No. 6,468,479, thedisclosure of which is incorporated herein by reference. The chlorinedioxide is generated either directly as a gas, or preferably as anaqueous (or other suitable liquid carrier) chlorine dioxide mixture. Thegenerator is preferably run using an excess of sodium chlorite to reducethe possibility of generating chlorine gas as an impurity. Othergenerally accepted methods for generating chlorine dioxide can be foundin, for example, U.S. Patent Pub. No. 2006/0068029 (U.S. patentapplication Ser. No. 11/131,021), the disclosure of which isincorporated herein by reference. Furthermore, the generator preferablyuses wastewater 15 as the drive fluid for generating chlorine dioxideand brings chlorine dioxide gas into contact with wastewater 15 under avacuum pressure such that the chlorine dioxide gas is drawn intowastewater 15 to form a chlorine dioxide aqueous solution.

In certain embodiments, the fluid to be treated is circulated through aclosed-loop system and treated in situ in accordance with the methodsand systems disclosed herein until the contaminants are oxidized and theappropriate residual of chlorine dioxide is established in vessel 50. Instill other embodiments, after treatment with chlorine oxide 30 andsecond oxidant 40, the treated fluids are allowed to stand in vessel 50for an appropriate period of time to allow the solids to settle and freeoil to be skimmed prior to reuse or discharge. In still otherembodiments, the fluid treated is used immediately after treatment forsubsequent crude oil, hydraulic fracturing, or other industrialapplications.

Furthermore, in alternative embodiments of the invention, the system orprocess disclosed herein may be combined with one or more traditional ornontraditional biocides, either oxidizing or non-oxidizing, to achieve asynergistic biocidal effect. Additionally, in alternative embodiments,one of ordinary skill in the art will readily appreciate that additionaltreatment processes known in the art can be incorporated in line orelsewhere in the system (either prior to treatment in accordance withthis invention, or subsequent thereto) in either batch or continuousoperation. By way of example only, and not meant to be limiting,treatment processes to remove oil and/or solids can be incorporated intothe system, or if foaming occurs, one might incorporate a chlorinedioxide compatible defoamer. Similarly, in certain embodiments, themethod and system disclosed herein can be added to, or retrofitted into,a preexisting recycling or treatment system. One of ordinary skill inthe art will also readily appreciate that in one or more embodiments,appropriate measurement and monitoring apparatus and/or equipment may beincorporated into the method and system disclosed herein.

In the embodiments disclosed herein, one of ordinary skill in the artwill appreciate that chlorine dioxide residual can be determined and/orcalculated using Method 4500-ClO2 E Amperometric Method II described inStandard Methods the Analysis of Water and Wastewater, or via modifiedversions of the same, wherein Standard Method 4500-ClO2 E AmperometricMethod II uses the following calculations:ClO₂ (mg/L)=1.25×(B−D)×0.00564×13,490/200Chlorite (mg/L)=D×0.00564×16,863/200Chlorine (mg/L)=[A−(B−D)/4]×0.00564×35,453/200,

where Titration A titrates the chlorine and one-fifth of the availablechlorine dioxide, Titration B titrates four-fifths of the chlorinedioxide and chlorite, Titration C titrates the non-volatilized chlorine(nitrogen gas purges the sample of the chlorine dioxide), but is notused in any calculation, and Titration D titrates the chlorite. In stillother embodiments, chlorine dioxide residual can be determinedspectrometrically or by measurement of oxidation reduction potential(ORP), each of which are incorporated herein, or via modified versionsof the same.

To facilitate a better understanding of the present invention, thefollowing examples of embodiments in accordance with the invention aregiven. It should be understood, however, that no limitation of the scopeof the invention is intended, and the following examples should not beread to limit or define the scope of the invention.

EXAMPLES

In the following examples, the effect of chlorine dioxide on oilfieldwastewater, with and without oxygen treatment, was studied.

Example 1

The following experiment was conducted to determine how significantlythe addition of air/oxygen affects chlorine dioxide (and/or chlorite)treatment of a sample of oilfield wastewater. The experimental resultsdemonstrate that the combination of air/oxygen with chlorine dioxide orchlorite has an unexpected, beneficial result of substantially reducingthe oxidant dosage required for oxidation of sulfides present inoilfield wastewater. Additionally, the combination of air/oxygen withchlorine dioxide unexpectedly achieves bacterial kill at significantlyreduced dosages. In contrast, air/oxygen addition alone is notsufficient over a reasonable period of time to remove sulfides fromwastewater or to kill bacteria present therein, and the addition ofalternative oxidants (i.e. nitrogen) do not have the same synergisticeffect.

For each of experiments 1(A)-1(G) below, a sample of water was used thatcontains 10 percent solids with 110 mg/l of sulfide in the aqueous phaseand has a pH of 8.2. The solids consist of biomass, inorganic material,hydrocarbon, and insoluble sulfides at a concentration of 82.5 mg/kg.Sulfide reducing and general aerobic bacteria were cultured from thesample, demonstrating growth over 10⁶ cfu/ml. The sample (solution andsolids) have a black coloration.

First, a series of experimental controls were conducted as follows:

Control A. A 200 ml portion of the sample was treated with 335 mg/lchlorine dioxide over a 15 minute period while stirring to achieve atrace (<1.0 mg/l) residual of chlorine dioxide in solution. The samplequickly turns from a black coloration to a brown/orange with theinsoluble solids settling quickly and an iron type floc forming. Therewas also a slight sheen of hydrocarbon on the surface of the treatedsample. No further change in appearance of the treated fluid wasobserved over 5 minutes. The solids (sludge) and fluid were analyzed forsulfide content using a Garret Gas Train. No detectable sulfides werefound in the solids or fluids. Sulfur reducing and general aerobicbacteria were cultured from the sample, demonstrating no bacterialgrowth.

Control B. A 200 ml portion of the sample was treated with 230 mg/lchlorine dioxide over a five minute period while stirring. The samplequickly turns from a black coloration to a grey brown/orange with theinsoluble solids settling quickly and an iron type floc forming. Nofurther change in appearance of the treated fluid was observed over 5minutes. The solids (sludge) and fluid were analyzed for sulfide contentusing a Garret Gas Train. There was 31 mg/l and 51 m/kg found in thefluid and sludge, respectively. No chlorine dioxide residual waspresent. Sulfur reducing and general aerobic bacteria were cultured fromthe sample, demonstrating bacterial growth over 10⁶ cfu/ml.

Control C. A 200 ml portion of the sample was treated with 420 mg/l ofchlorite (560 mg/l as sodium chlorite) while stirring. The sample turnsfrom a black coloration to a brown/orange with the insoluble solidssettling and an iron type floc forming over a ten minute period. Therewas also a slight sheen of hydrocarbon on the surface of the treatedsample. No further change in appearance of the treated fluid wasobserved after 10 minutes. The solids and fluid were analyzed forsulfide content using a Garret Gas Train. No detectable sulfides werefound in the fluids, however the solids contain approximately 15 mg/lsulfide. Sulfur reducing and general aerobic bacteria were cultured fromthe sample, demonstrating growth over 10⁶ cfu/ml.

Control D. A 200 ml portion of the sample was sparged with air through afine diffuser stone at a rate of 2 SLPM for 30 minutes. Over the 30-minperiod, the sample turns from a black coloration to a grey coloration.The solids (sludge) and fluid were analyzed for sulfide content using aGarret Gas Train. The fluid contains 60 mg/l sulfide and the solidscontain 75 mg/l sulfide. Sulfur reducing and general aerobic bacteriawere cultured from the sample, demonstrating growth over 10⁶ cfu/ml.

Sparging experiments were then conducted in three systems (air-chlorinedioxide, nitrogen-chlorine dioxide, and air-chlorite) as follows:

Experiment E. A 200 ml portion of the sample was sparged with airthrough a fine diffuser stone at a rate of 2 SLPM for four (4) minutes.Initiated concurrently, a dose of 230 mg/l chlorine dioxide was addedover a five (5) minute period, with the last minute of dosing beingadded without air sparging. In this example, ClO2 is added at a lowenough rate with a volume and flow rate of air that does not strip thechlorine dioxide before it reacts. The sample quickly turns from a blackcoloration to a brown/orange with the insoluble solids settling quicklyand an iron type floc forming upon the cessation of sparging. No furtherchange in appearance of the treated fluid was observed over five (5)minutes. The solids (sludge) and fluid were analyzed for sulfide contentusing a Garret Gas Train. There were no detectable sulfides in thesolids or fluid. Sulfur reducing and general aerobic bacteria werecultured from the sample, demonstrating no bacterial growth.

Experiment F. A 200 ml portion of the sample was sparged with nitrogenthrough a fine diffuser stone at a rate of 2 SLPM for four (4) minutes.Initiated concurrently, a dose of 230 mg/l chlorine dioxide was addedover a five (5) minute period, with the last minute of dosing beingadded without nitrogen sparging. The sample quickly turns from a blackcoloration to a brown/orange with the insoluble solids settling quicklyand an iron type floc forming upon the cessation of sparging. No furtherchange in appearance of the treated fluid was observed over 5 minutes.The solids (sludge) and fluid were analyzed for sulfide content using aGarret Gas Train. There were 7 mg/l and 160 mg/l sulfides remaining inthe fluid and the solids, respectively. Sulfur reducing and generalaerobic bacteria were cultured from the sample, demonstrating over 10⁶bacterial growth.

Experiment G. A 200 ml portion of the sample was sparged with airthrough a fine diffuser stone at a rate of 2 SLPM for 15 minutes.Initiated concurrently, a dose of 300 mg/l of chlorite (402 mg/l assodium chlorite) was added over a five (5) minute period. The sampleturns from a black coloration to a brown/orange with the insolublesolids settling quickly and an iron type floc forming upon the cessationof sparging. No further change in appearance of the treated fluid wasobserved over 15 minutes. The solids (sludge) and fluid were analyzedfor sulfide content using a Garret Gas Train. There were no detectablesulfides in the solids or fluid. Sulfur reducing and general aerobicbacteria were cultured from the sample, demonstrating bacterial growthover 10⁶ cfu/ml.

In the following examples, the unexpected, synergistic effect oftreating a storage tank with oilfield wastewater with a treatment ofchlorine dioxide and oxygen in a closed loop system was studied.Sparging experiments were conducted on two systems (air-chlorine dioxideonly, and air-chlorite-chlorine dioxide) as follows:

Example 2

A tank containing about 30,000 barrels (bbl) of produced fresh and flowback water was analyzed and found to contain 16,000 mg/l TDS, over 10⁶cfu/ml bacteria, and 40 mg/l sulfides in the homogenized fluid at a pHof 7.8. The chlorine dioxide demand of the fluid to be treated wasdetermined to be 180 mg/l. The amount of 50% sodium hydroxide requiredto maintain the pH was determined to be 630 gallons.

The tank was rigged to a chlorine dioxide generator (see, e.g. U.S. Pat.No. 6,468,479). Although not limiting, one example of generator would bea Sabre BB series portable DiKlor® generation system with a maximumcapacity of 24,000 lbs. per day continuous production. This system isself-contained and has a distribution system that allows it to circulatefluids in the tank. More specifically, a drive fluid stream waswithdrawn from the tank and circulated through a chlorine dioxidegenerator by means of a centrifugal pump at a rate of 320 gallons perminute. The generator is arranged so that the suction for the drivefluid stream is pulled from the lowest end of the tank, and thedischarge solution containing chlorine dioxide and/or air was returnedto the tank and discharge to the bottom of the tank via a movableinjection boom. The injection boom was continuously moved around thetank at a rate of 50 feet per minute.

Sodium hydroxide was added to the tank with enough sodium chlorite toabsorb approximately 10 percent of the theoretical chlorine dioxidedemand. In this specific example, and in accordance with calculationsreadily known in the art, the amount of sodium chlorite required toabsorb 10% of the chlorine dioxide demand was a dosage of approximately23 mg/l chlorite. The sodium hydroxide and chlorite were added over asixty minute period with air at a rate of 125 SCFM. In this embodiment,air was introduced via a venturi. At the end of the 60-minute period,the injection of air is discontinued, and chlorine dioxide demand wasretested and found to be 27 mg/l. Chlorine dioxide then was introducedvia a venturi at an appropriate rate to achieve a dosage of 47 mg/l overa 30 minute period. No air was introduced during the chlorine dioxidestep.

The resulting fluid was clear with orange/brown sediment and had a thinlayer of floc on top that was determined to be 98% inorganic materialand 2% hydrocarbons. 8 mg/l chlorine dioxide was found as a residual inthe fluid. The fluid, sludge, and floc were analyzed by garret gas trainand determined to contain no sulfides. No bacterial growth was found byculture analysis. The fluid was analyzed to determine suitability for“gelling” for fracturing use. The fluid gelled and cross linked withoutdifficulty. This method resulted in a 75% reduction in the amount ofchlorine dioxide required to achieve the target chlorine dioxideresidual and no bacterial grown.

Example 3

A tank containing about 30,000 bbl of produced fresh and flow back waterwas analyzed and found to contain 16,000 mg/l TDS, over 10⁶ cfu/mlbacteria, and 40 mg/l sulfides in the homogenized fluid at a pH of 7.8.The chlorine dioxide demand of the fluid was determined to be 180 mg/l.The tank was rigged to a chlorine dioxide generator where the suctionfor a drive fluid stream is pulled from the lowest end of the tank, andthe discharge solution containing chlorine dioxide and/or air wasreturned to the tank and discharge to the bottom of the tank via amovable injection boom. The injection boom was continuously moved aroundthe tank at a rate of 50 feet per minute. The fluid was withdrawn fromthe tank and circulated through the chlorine dioxide generator by meansof a centrifugal pump at a rate of 320 gallons per minute.

In this example, chlorite was not added directly to the system as sodiumchlorite. Instead, chlorine dioxide was added to the tank initially(which converted to chlorite), followed by air and then a second dosageof chlorine dioxide as set forth below. More specifically, 1) from timezero (0) and over the first 10 minutes, chlorine dioxide was added toprovide 20% of the total dosage; 2) from minute 10 through minute 30,the solution was circulated; 3) from minute 30 to minute 60, air wasadded; and 3) from minute 60 through minute 80, the remaining 80% of thechlorine dioxide was introduced into the tank. In total, the tank wastreated with 110 mg/l chlorine dioxide over an aggregate (but,nonconsecutive) 50-minute period. In regards to step 2, one of ordinaryskill in the art will recognize that, when a big tank is used, one hasto be careful not to get localized “hot spots” and allow the chlorinedioxide to disperse a bit.

In step 3, air was added in isolation through a venturi at a rate of 100SCFM to tank 250 from minute 30 to minute 60. In alternate embodiments,one could add low dosages of ClO2 with air, depending on the size anddepth of the vessel, as well as the flow rate. Sodium hydroxide wasadded concurrently to maintain stable pH. The fluid was analyzed posttreatment. The resulting fluid was clear with orange/brown sediment andhad a thin layer of floc on top that was determined to be 98% inorganicmaterial and 2% hydrocarbons. 12 mg/l chlorine dioxide was found as aresidual in the fluid. The fluid, sludge, and floc were analyzed bygarret gas train and determined to contain no sulfides. No bacterialgrowth was found by culture analysis. The fluid was analyzed todetermine suitability for “gelling” for fracturing use. The fluid gelledand cross linked without difficulty. This method resulted in about a 40%reduction in the amount of chlorine dioxide required to achieve a targetchlorine dioxide residual and no bacterial grown.

Example 4

A tank contained 4200 gallons of produced water. The homogenized fluidwas analyzed and found to contain 23,000 mg/l TDS, over 104 cfu/mlbacteria, and 175 mg/l sulfides with a pH of 7.8. The chlorine dioxidedemand of the fluid was determined to be 580 mg/l. The tank was riggedto a chlorine dioxide generator where the suction for the drive fluidstream is pulled from the lowest end of the tank, and the dischargesolution containing chlorine dioxide and/or air via a perforated pipealong the length of the bottom of the tank. The fluid is withdrawn fromthe tank and circulated through the chlorine dioxide generator by meansof a centrifugal pump at a rate of 320 gallons per minute.

As in Example 3, chlorine dioxide was added to the tank initially via aventuri 320, followed by air and then a second dosage of chlorinedioxide as set forth below. More specifically, 1) from time zero (0) andover the first minute, chlorine dioxide was added to provide 20% of thetotal dosage; and then 2) from minute six (6) through minute ten (10),the remaining 80% of the chlorine dioxide was introduced into the tank.In total, the tank was treated with 310 mg/l chlorine dioxide over anaggregate (but, nonconsecutive) 5-minute period. Air was added through aventuri at a rate of 50 SCFM to the tank from minute one (1) to minutesix (6). Sodium hydroxide was added concurrently to maintain stable pH.The fluid was analyzed post treatment. The fluid was clear withorange/brown sediment and had a thin layer of floc on top that wasdetermined to be 96% inorganic material and 4% hydrocarbons. 7 mg/lchlorine dioxide was found as a residual in the fluid. The fluid,sludge, and floc were analyzed by garret gas train and determined tocontain no sulfides. No bacterial growth was found by culture analysis.This method resulted in about a 47% reduction in the amount of chlorinedioxide required to achieve a target chlorine dioxide residual and nobacterial grown.

Example 5

A tank contained 4200 gallons of produced water. The homogenized fluidwas analyzed and found to contain 23,000 mg/l TDS, over 104 cfu/mlbacteria, and 175 mg/l sulfides with a pH of 7.8. The chlorine dioxidedemand of the fluid was determined to be 580 mg/l. The tank was riggedto a chlorine dioxide generator where the suction for the drive fluidstream is pulled from the lowest end of the tank, and the dischargesolution containing chlorine dioxide and/or air via a perforated pipealong the length of the bottom of the tank. The fluid is withdrawn fromthe tank and circulated through the chlorine dioxide generator by meansof a centrifugal pump at a rate of 320 gallons per minute.

In this example, chlorite was introduced directly at a rate to achieve adosage of 120 mg/l over the first minute. Chlorine dioxide also wasadded at a rate to achieve a dosage of 210 mg/l over an aggregate five(5) minute period. Specifically, chlorine dioxide was added from timezero (0) to minute one (1), and then again from minute six (6) to minuteten (10). Air was added through a venturi at a rate of 50 SCFM to thetank from minute zero to minute nine. Sodium hydroxide was addedconcurrently to maintain stable pH. The fluid was analyzed posttreatment.

The treated fluid was clear with orange/brown sediment and had a thinlayer of floc on top that was determined to be 96% inorganic materialand 4% hydrocarbons. 7 mg/l chlorine dioxide was found as a residual inthe fluid. The fluid, sludge, and floc were analyzed by garret gas trainand determined to contain no sulfides. No bacterial growth was found byculture analysis. This method resulted in about a 43% reduction in theamount of chlorine oxides required to achieve a target chlorine dioxideresidual and no bacterial grown.

Although the Examples and descriptions above discuss what is, inessence, a closed loop treatment system, the systems and methodsdisclosed herein and claimed could also be utilized for a frac “on thefly” system and method, wherein the treated water would be usedimmediately and/or shortly after being treated for fracturing. Forexample, in one embodiment, the frac water to be injected into thesubterranean formation would be treated using the methods disclosedherein out in the oilfield, ahead of the well head. For this system, youwould continuously be filling a vessel (e.g. onsite frac tanks, locatedat the frac site/oilfield) with source water that needs to be treatedprior to introduction into the well. The water could comprise surfacewater, municipal water, produced water, flow back water, or anycombination of the above (“commingled water”).

While the preferred application for the method and system disclosedherein is in the oil field applications, such as petroleum wells,downhole formations, and industrial and petroleum process water,additional industrial applications include, but are not limited to,cooling water systems, mineral process waters, geothermal wells, papermill digesters, washers, bleach plants, stock chests, and white watersystems, black liquor evaporators in the pulp industry, continuouscasting processes in the metallurgical industry, air conditioning andrefrigeration systems, water reclamation systems, water purificationsystems, membrane filtration systems, food processing streams (meat,vegetable, sugar cane, poultry, fruit and soybean); and waste treatmentsystems as well as clarifiers, municipal sewage treatment, municipalwater systems, potable water systems, aquifers, and water tanks.

Various embodiments and modifications of this invention have beendescribed in the foregoing description. Such embodiments andmodifications are illustrative only and are not to be taken as limitingin any way the scope of the invention, which is defined by the followingclaims. Other variations of what has been described also fall within thescope of the invention, and the present invention may be modified andpractices in different but equivalent manners apparent to those skilledin the art having the benefit of the teachings herein. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. All numbers andranges disclosed above may vary by some amount. Also, the terms in theclaims shall have their plain, ordinary meaning unless otherwiseexplicitly and clearly defined by the patentee. Subject matterincorporated by reference is not considered to be an alternative to anyclaim limitations, unless otherwise explicitly indicated.

The invention claimed is:
 1. A method for reducing, inactivating,destroying, removing, or eliminating from an aqueous fluid one or morecontaminants selected from the group consisting of sulfur compounds,bacteria and combinations thereof, the method comprising the steps of(i) introducing an oxidant into the aqueous fluid at a flow rate thatavoids off-gassing of volatile reductants from the aqueous fluid, (ii)introducing a first chlorine oxide comprising sodium chlorite into theaqueous fluid either prior to or contemporaneous with the step ofintroducing the oxidant, and (iii) introducing a second chlorine oxidecomprising chlorine dioxide into the aqueous fluid after introducing thefirst chlorine oxide and oxidant, wherein said oxidant is selected fromthe group consisting of oxygen (O₂), air, oxygen (O₂ )-enriched air, andcombinations thereof, and is substantially ozone-free.
 2. The method ofclaim 1 further comprising introduction of sodium hydroxide into theaqueous fluid to maintain stable pH.
 3. The method of claim 1 whereinthe second chlorine oxide is an aqueous chlorine dioxide solution. 4.The method of claim 3 further comprising the step of generating thechlorine dioxide aqueous solution by using a portion of the aqueousfluid to be treated.
 5. The method of claim 3, wherein the aqueoussolution of chlorine dioxide is produced by flushing chlorine dioxidegas into water, wherein chlorine gas is present in the flushed gas at alevel less than 5%.
 6. The method of claim 5, wherein chlorine gas ispresent in the flushed gas at a level less than 0.5%.
 7. The method ofclaim 1, wherein the residual concentration of chlorine dioxide in theaqueous fluid after treatment is in the range of about 0.1 mg/l to about50 mg/l.
 8. The method of claim 1 wherein the source of the aqueousfluid is an aqueous fluid stream; a vessel, tank, pit, lagoon, or pondfor storing waste water; a water treatment plant; a hydraulic fracturingtank; or a piece of equipment, pipeline or vessel used for hydraulicfracturing or crude oil production.
 9. The method of claim 1, whereinthe residual concentration of chlorine dioxide in the aqueous fluidafter treatment is at least 0.1 mg/l.
 10. The method of claim 9, furthercomprising measuring the residual concentration of chlorine dioxide inthe aqueous fluid after treatment.
 11. The method of claim 1, whereinthe method reduces the total amount of chlorine oxide required toachieve a target residual concentration of chlorine dioxide in theaqueous fluid.
 12. The method of claim 1, wherein the chlorine dioxidedemand of the aqueous fluid is reduced by at least 10% prior tointroducing the second chlorine oxide into the aqueous fluid.
 13. Themethod of claim 1, wherein introducing the oxidant and the firstchlorine oxide reduces by at least 40% the total amount of chlorinedioxide required to achieve a target residual concentration of chlorinedioxide in the aqueous fluid, wherein the target residual concentrationof chlorine dioxide is at least 0.1 mg/ml.
 14. The method of claim 13,wherein the target residual concentration of chlorine dioxide is 0.1mg/l to about 50 mg/l.
 15. The method of claim 13, wherein the oxidantis introduced by air sparging, a pressurized source, an aerator,mechanical agitation, a diffuser, spraying, or eduction via a venturi.16. A method for reducing, inactivating, destroying, removing, oreliminating from an aqueous fluid one or more contaminants selected fromthe group consisting of sulfur compounds, bacteria and combinationsthereof, the method comprising the steps of (i) introducing an oxidantinto the aqueous fluid at a flow rate that avoids off-gassing ofvolatile reductants from the aqueous fluid, wherein said oxidant isselected from the group consisting of oxygen (O₂), air, oxygen(O₂)-enriched air, and combinations thereof and is substantiallyozone-free; (ii) introducing a first chlorine oxide into the aqueousfluid either prior to or contemporaneous with the step of introducingthe oxidant, wherein said first chlorine oxide is selected from thegroup consisting of chlorine dioxide, chlorite, and combinationsthereof; and (iii) introducing a second chlorine oxide into the aqueousfluid after introducing the oxidant and the first chlorine oxide,wherein the second chlorine oxide is chlorine dioxide, wherein thechlorine dioxide demand of the aqueous fluid is reduced by at least 10%prior to introducing the second chlorine oxide into the aqueous fluid.17. The method of claim 16, wherein the oxidant is introduced by airsparging, a pressurized source, an aerator, mechanical agitation, adiffuser, spraying, or eduction via a venturi.
 18. The method of claim16, wherein introducing the oxidant and the first chlorine oxide reducesthe total amount of chlorine oxide required to achieve a target residualconcentration of chlorine dioxide in the aqueous fluid.
 19. The methodof claim 16, wherein introducing the oxidant and the first chlorineoxide reduces by at least 40% the total amount of chlorine dioxiderequired to achieve a target residual concentration of chlorine dioxidein the aqueous fluid, wherein the target residual concentration ofchlorine dioxide is at least 0.1 mg/ml.
 20. The method of claim 19,wherein the target residual concentration of chlorine dioxide is 0.1mg/l to about 50 mg/l.
 21. The method of claim 16, wherein sodiumhydroxide is introduced into the aqueous fluid prior to the step ofintroducing the oxidant, either in combination with the first chlorineoxide or as a separate feed substantially contemporaneously therewith.22. The method of claim 16, wherein the aqueous fluid has a chlorinedioxide demand of about 30 mg/l to about 5000 mg/l prior to treatment.23. A method for reducing, inactivating, destroying, removing, oreliminating from an aqueous fluid one or more contaminants selected fromthe group consisting of sulfur compounds, bacteria and combinationsthereof, the method comprising the steps of (i) introducing an oxidantinto the aqueous fluid at a flow rate that avoids off-gassing ofvolatile reductants from the aqueous fluid, wherein said oxidant isselected from the group consisting of oxygen (O₂), air, oxygen(O₂)-enriched air, and combinations thereof and is substantiallyozone-free; (ii) introducing a first chlorine oxide into the aqueousfluid either prior to or contemporaneous with the step of introducingthe oxidant, wherein said first chlorine oxide is selected from thegroup consisting of chlorine dioxide, chlorite, and combinationsthereof; and (iii) introducing a second chlorine oxide into the aqueousfluid after introducing the oxidant and the first chlorine oxide toachieve a target residual concentration of chlorine dioxide in the rangeof about 0.1 mg/ml to about 50 mg/l in the aqueous fluid, wherein thesecond chlorine oxide is chlorine dioxide, wherein the introduction ofthe oxidant and the first chlorine oxide reduces by at least 40% thetotal amount of chlorine dioxide required to achieve the target residualconcentration of chlorine dioxide.
 24. The method of claim 23, whereinthe aqueous fluid has a chlorine dioxide demand of about 30 mg/l toabout 5000 mg/l prior to treatment.
 25. The method of claim 23, whereinthe chlorine dioxide demand of the aqueous fluid is reduced by at least10% prior to introducing the second chlorine oxide into the aqueousfluid.
 26. The method of claim 23, wherein the oxidant is introduced byeduction via a venturi.